Exposed Stepper Motor Windings

Got a stepper motor from halfway around the planet from the usual eBay source, intended for a direct-drive extruder (at some point). This one has integral wire leads, which is fine with me, but the opening in the rear endcap reveals a bit more of the innards than one usually sees:

ACT 17HS5425 stepper - exposed winding

Yup, that’s one winding peeking out. Although the wire insulation should take care of anything conductive, I’d expect the same casual attention to detail in the winding terminals.

I’d worry more if this were being used in a metal-cutting operation, but a snippet of heatshrink tubing and a blob of hot-melt glue seem in order.

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This entry was posted on 2011-08-19, 07:24 and is filed under Electronics Workbench, Machine Shop. You can follow any responses to this entry through RSS 2.0.
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You probably already know this, but just to be on the safe side: don’t take the motor apart (or at least, do not remove the rotor) to fix it. A dab of hotglue from the outside should be fine, even removing the end-caps may be fine… but when the rotor gets removed from the stator, it loses part of its fieldstrength. It will then never again achieve its torque specifications.

Surprized there’s also no extra insulating sleeve around the wires as they exit the motor. Just a bit of extra strain relief and mechanical protection I like to see on them….

Yup, I’ve learned that from the mistakes of others… the motor seems to work when you put it back together again, but the magnetism falls right out of the rotor and the torque will never be the same again.

The MBI Stepstruder motor has trouble under less-than-ideal conditions, mostly because of its high winding resistance and huge inductance. The low-speed torque works out around 500 mNm at 805 mA, which I think is optimistic due to the big time constant and high winding voltage.

The 7:51 gearing on my extruder probably produces 700-ish mNm at the shaft from 100 mNm at the motor, which seems grossly excessive. It nigh onto sucked the filament spool inside out before skipping steps, sooo …

This 48 mm motor is on the order of 450 mNm, so it’s probably pretty close to what’s needed. It’ll dissipate 7.8 W at that current, which is crazy high for something twisting a plastic mounting, though. The Stepstruder runs around 8.7 W (with the proper current setting) and requires a fan.

I’ve never seen any convincing torque numbers for the MK5 hot end, anyway, so I think it’s up for grabs. Nophead produced some numbers a while ago, for a very different extruder design, but that’s about it.

What’s the minimum amount of torque you’ve found to be required to feed standard 3mm filament? if 700 mNm is excessive, what would be too little? I’m mostly wondering how small of a motor you could go with if you paired it with a good enough gearbox. Obviously going with a smaller motor and a gearbox loses you the ability to extrude super-quickly, but that’s not currently very realistic anyway.

Also, would you happen to know the relationship between the RPM of the pinch wheel and the flow rate?

I know it’s more than 100-ish mNm, because a 34-ish mm NEMA 17 motor isn’t enough for direct drive at useful speeds. Something around 300 mNm seemed workable for a RepRap with a different hot end. My geared stepper can squirt plastic at more than 5 rpm, although it runs out of heating capacity pretty quickly.

Those are the boundaries I know about; there’s surely more information I don’t know!

I ask because just for fun, I’ve decided to design a geared extruder with a tiny NEMA 8 motor I picked up. The thing can output 40 mNm, so I’m planning to downshift it 5:1 and start at 200 mNm and see what happens. We’ll see how slow I have to go and whether or not a speed that works (if it ever works) is usable for extrusion. I also have both 3mm and 1.75mm filament, so I should be able to actually test the notion that 1.75mm filament requires less torque.

That seems a bit low, but it’s certainly worth investigating. For forward extrusion you need only turn the extruder at 2 rpm, which is certainly do-able with enough geardown.

The gotcha will be getting sufficient reverse motion speed with a high gear ratio: Reversal requires very fast motion with no acceleration limiting. Dimension adds in acceleration (I think), but you still need fairly high speeds.

Back of the envelope: Geardown 8:1 to get 320 mNm with 30 rpm reversal = 240 rpm = 4 rev/s = 800 full step/s. The torque for a Minebea NEMA 10 motor (for which I have the doc) falls off a cliff at 1000 step/s, so if it doesn’t work you’re stuck: more geardown = more speed = much less torque = no biscuit.

Even less than that: the filament rises up out of the extruder all by itself without any help from the motor!

However, the un-reversal motion requires building up full extruder pressure at high speed. The first part is easy, but ramming that last millimeter or so into the wad of goo behind the nozzle will be challenging.

One a day, every day…

Redshifting

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